A catalyst is a substance that increases the rate of a chemical reaction without, itself, being consumed in the reaction. Catalyzed reactions proceed through a mechanism that is not apparent in the stoichiometry of the reaction. For example, catalysts may be used in the rate-determining step of a reaction and later re-formed, so there is no net change in the concentration of the catalyst during the reaction. Specifically, catalysts lower the activation energy associated with the rate-determining step which accelerates the chemical reaction. Many types of materials may function as catalysts for different types of reactions.
Catalysts may be used in numerous applications. For example, a fuel cell is an electrochemical device that utilizes catalysts. Typically, a fuel cell includes an anode, a cathode, and a solid or liquid electrolyte therebetween. Fuel materials (e.g., hydrogen, fossil fuels, and small organic molecules) are brought in contact with the anode, and an oxidizing gas (e.g., oxygen and air) is brought in contact with the cathode. The fuel is oxidized in a chemical reaction which may be accelerated by the presence of a catalyst at the anode. The oxidizing gas is reduced in a chemical reaction which also may be accelerated by a catalyst at the cathode. The cell generates electricity when electrons generated in the fuel oxidation reaction at the anode flow through an external circuit to the cathode where the electrons are consumed in the reduction reaction.
A number of different materials have been investigated for use as catalysts in fuel cell electrodes (i.e., anodes and cathodes). In particular, anodes in cells that utilize small organic molecule fuels (e.g., methanol, formic acid, ethanol, and ethylene glycol, amongst others) must satisfy a number of property requirements. For example, in these fuel cells the anode must sufficiently catalyze the oxidation reaction, while minimizing the strong binding of CO, which is often produced in intermediate reactions, to the surface. This so-called “CO poisoning” can limit the catalytic activity of the anode over time which can severely impair cell maximum output power and efficiency.
Pure transition metals (e.g., Pt, Pd, Ni) have been used as electrocatalysts in small organic molecule fuel cells because of their high catalytic activity for these fuels. However, these metals, and particularly Pt, readily become poisoned with CO thus limiting cell performance over time. CO poisoning is particularly problematic at lower temperatures (e.g., less than 200 degrees C.).
Surface modified transition metal electrodes have also been investigated in small organic molecule fuel cells. Surface modification involves adsorbing monolayer amounts of metal adatoms (e.g., Bi) on the surface of the electrode. The metal adatoms may be selected from groups of metals that weakly bind oxygen, which is necessary for oxidation reactions, thus, enhancing the oxidation of CO to CO2 and mitigating CO poisoning. However, the surface composition of surface modified electrodes may change over time as the metal adatoms react with impurities, desorb, or otherwise migrate from the electrode surface. Thus, CO poisoning can increase over time and may compromise cell performance.
Alloys of transition metals (e.g., Pt—Ru, Pt—Rh) have also been used as electrodes in small organic molecule fuel cells. However, while such alloys may perform better than Pt electrodes, they still may be susceptible to CO poisoning effects. Moreover, the composition of such alloys may change over time as species (e.g., Ru, Rh) migrate from the surface into the bulk to leave primarily the other alloy component (e.g., Pt) at the surface, which can be more susceptible to CO poisoning.
CO poisoning may also prevent hydrogen gas that includes relatively high levels of CO (e.g., greater than about 0.01 mole percent) from being used as a fuel in conventional hydrogen fuel cells that operate at relatively low temperatures (e.g., 80 degrees C. or less). The rapid adsorption of CO on electrode surfaces of these cells when oxidizing hydrogen gas, which includes high levels of CO, rapidly reduces the maximum output power and efficiency of such cells to unacceptable levels. Hydrogen gas that is produced by processing hydrocarbons (e.g., natural gas) in a reformer typically includes between about 3 and 10 mole percent CO. Therefore, such hydrogen gas must be further processed (e.g., using water gas shift reactors and/or preferential oxidizers) to reduce the CO concentration to acceptable levels (typically less than 0.001 mole percent) prior to using in conventional fuel cells at low temperatures.